1. Field of the Invention
This invention relates to integrated circuit amplifiers, and more particularly to amplifiers which are employed as the output stage of another circuit.
2. Description of the Prior Art
Various amplifier circuits have been used as the output stage of comparators and other circuit devices. These amplifiers have generally been incapable of obtaining a desirable combination of high current gain, wide operating range and fast response time.
A simple output amplifier stage employed in the prior art is shown in FIG. 1. In this circuit the output from a previous stage is delivered to an input terminal 2 which is connected to the base of transistor Q1. A current source I1 supplies current from a positive voltage bus V+ through the collector-emitter circuit of Q1 to ground potential, and also to the base of another transistor Q2. The collector of Q2 is connected through a resistor R1 to V+, and also feeds an output terminal 4; the emitter of Q2 is grounded. R1 is typically an external resistor supplied by the user.
The circuit of FIG. 1 provides a single stage of amplification, with the output current at terminal 4 limited to the current gain of Q2 (Beta) times I1. It has a relatively slow response time because of saturation problems associated with Q2.
FIG. 2 illustrates another prior art circuit which solves the saturation problems of the FIG. 1 circuit, and can therefore operate at a higher speed. Its basic limitation is that its output current is still limited to Beta times I1. The FIG. 2 circuit, commonly referred to as a "Schottky clamped output stage", adds to the circuitry of FIG. 1 a Schottky-barrier diode D1 which is connected between the collector of Q1 and the junction between the collector of Q2 and output terminal 4, the diode being oriented to conduct current toward the junction. A Schottky-barrier diode, also known as a hot-carrier or a surface-barrier diode, consists of a rectifying metal-to-semiconductor contact rather than an npn junction. When properly fabricated, such a rectifying contact minimizes charge-storage effects and increases diode switching speed by shortening turn-off time. Schottky-barrier diodes exhibit a smaller forward voltage drop for low current levels than do standard junction diodes, typically in the range of 400-450 mV for Schottky-barrier diodes as opposed to 650-700 mV for standard junction diodes.
Except for the addition of the Schottky-barrier diode, the prior art FIG. 2 circuit is identical to that of FIG. 1. While the Schottky-barrier diode diverts enough current from the base of Q2 to keep it from saturating, and thereby increases the operating speed of the circuit, the output current is still limited to Beta times I1, as noted above.
A third popular output amplifier stage, commonly referred to as a direct-coupled Darlington amplifier, is illustrated in FIG. 3. In this circuit a second amplifying stage, consisting of transistor Q3 with its collector connected to the collector of Q2, its base connected to the emitter of Q2 and its emitter grounded, is used. A resistor R2 may be connected between the base of Q3 and ground. This circuit produces a significantly greater amplification, with the output current in the order of (Beta).sup.2 times I1. However, the operating range of the circuit is limited, since Q2 will saturate if the output voltage swings close enough to ground so that the Q3 base voltage exceeds its collector voltage.